A critical component of decision-making involves assessing the value between different options. Altered decision-making processes are a characteristic of many neurological disorders, including drug addiction and pathological gambling. Therefore, it is imperative to identify the neural systems mediating changes in how one assesses the value between different rewards. This proposal tests the hypothesis that the mesolimbic dopamine system is required for changing the value and relative preference between quantifiably identical food rewards that differ only in flavor. To address this question it is necessary to 1) manipulate the subjective preference between distinct rewards, 2) record dopamine release while the subjective preference changes, and 3) control the activity the dopamine system. This project employs a novel training paradigm in which the initial subjective preference between different flavors of food can either be modulated or maintained. The experiments in Aim 1 will utilize fast-scan cyclic voltammetry in rats to identify changes in reward-evoked dopamine release as the subjective preference evolves across training. The experiments in Aim 2 involve optogenetic manipulations of dopamine neurons at the time of reward delivery to establish the necessity and sufficiency of dopamine in controlling subjective preference. Together, the findings from these proposed experiments will elucidate the role of mesolimbic dopamine in changing the subjective valuation of rewards.